The present invention relates generally to the manufacture of integrated circuit lead frames. More particularly, it relates to a process for coating these lead frames to enhance the bonding of their internal leads to bonding wires connected to an integrated circuit (IC) and the solderability of their external leads to a circuit board.
Lead frames are the standard means for connecting a microscopic integrated circuit to outside circuitry and are usually made of copper. Typically, a die or chip containing the integrated circuit is attached to a bonding pad of a lead frame. Once the chip is attached, wires are bonded to input/output pads of the integrated circuit and to internal leads of the lead frame. This arrangement of a chip, a lead frame, and bonding wires is then encapsulated in a plastic casing leaving external leads of the lead frame exposed (outside of the plastic casing). The packaged integrated circuit can then be connected to other electronic components on a conventional circuit board by its external leads.
Integrated circuits have a multitude of uses. The packaged IC's themselves and the macroscopic circuit boards in which they are soldered are mass produced. Therefore, improvements in the manufacture and processing of the components of the packaged IC can have broad implications.
Lead frames plated with solder can have distinct advantages over unplated ones. By providing the solder directly on the lead frame, soldering of the leads to circuit boards is more reliable. Similarly, the process of bonding wires to the internal leads can be improved by plating the bonding areas of the internal leads with appropriate materials.
One lead frame technology has been described in European Patent Application 89302939.7 and is diagrammed in FIG. 1. This process 200 starts with lead frames being payed out 202 by a reel. Subsequently, they are degreased and cleaned at a step 204. After a simple washing, they are electrocleaned in a caustic alkaline solution. By then bathing the frame in a pickling acid, the base solution is neutralized, non-bound copper is removed, and a rough clean surface suitable for plating is formed. Finally, the lead frame is rinsed before plating.
The clean base frame is then plated overall with nickel at a step 206 by a conventional flood plating process. The initial nickel plating is followed by a palladium/nickel coating step 208. This in turn is followed by a nickel flood plating at a step 210 and then a palladium flood plating at a step 212 resulting in a lead frame having four coatings over the entire base copper frame. To complete the processing, the lead frames are cleaned, rinsed, and dried 214 before finally being wound 216 on a take-up reel.
A variation of this four-layer process is described in European Patent Application 90300934.8. In that process, a two-layer lead frame is made by first applying nickel and then palladium. A further variation is described in U.S. Pat. 4,628,165. In that process, a three-layer frame is formed by plating with nickel, followed by palladium/nickel, followed by palladium.
The above palladium layer approaches have been promoted as having good manufacturability. However, the solderability of the external leads of the processed frames, the primary reason for pre-plating, is quite questionable unless very corrosive fluxes are used. Texas Instruments has attempted the cumbersome technique outlined in European Patent Application 90300934.8 of dipping in or plating with a tin/lead alloy to enhance solderability. This step is performed after the lead frames have been cut from the reel into strips, assembled as strips of IC devices, then hung on racks and dipped or plated with tin/lead. It is known in the art as a cut-strip plating process. Thus, this post-plating of tin/lead is not part of the continuous process and suffers from the disadvantages inherent therein.
A state of the art technique for pre-plating lead frames is the Dyna-Craft PPF-3000, pro-plated frame, technology. An early version of PPF-3000 is described in U.S. Pat. 4,486,511. One version of PPF-3000 utilizes tin/lead platings on a copper lead frame base. A flow chart of the PPF-3000 manufacturing process 220 is shown in FIG. 2.
In this process, as in most continuos processes, lead frames undergo much of their processing in reel to reel form. After being payed out at step 222 by a reel, they must be degreased and cleaned at a step 224 just as in step 204 of the process described in FIG. 1. The clean base frame is then plated overall 226 with a nickel coating of about 30 to 60 microinches. This layer serves as a diffusion barrier to prevent diffusion of the copper in the lead frame base into a subsequently applied tin layer. Copper is known to rapidly diffuse into tin and tin alloys at standard integrated circuit packaging temperatures and thereby degrade solderability.
Copper is subsequently spot plated in step 228 on the inner leads and die attach pad over the coat of nickel by a spot plating. Spot plating technology for lead frames appears in the three U.S. Pat. Nos. 4,405,432, 4,404,079 and 4,404,080. Next, silver is spot plated 230 to aid the bonding of wires from the chip and improve the electrical conductivity and bondability at the wire bond junctions. The copper plating acts as a metallic glue to attach the silver to the nickel and to enhance the adhesion of the plastic encapsulation to the lead frame. To improve solderability to external circuitry, a tin/lead compound is spot plated 232 on the external leads. The finished pre-plated frame is then rinsed and dried 234 and finally wound 236 onto a take-up reel.
Although PPF-3000 represented a major advance in simplified IC packaging technology, and PPF-3000 frames sail through all solderability tests, the process has a few drawbacks. For example, its multiple spot platings 228, 230, and 232 are equipment intensive and may require frequent adjustments and careful positional alignment of the locations of the edges or boundaries of the plated spots (sometimes referred to as registration requirements) of the processing apparatus.
Also, a visible boundary of the tin/lead coat must be covered by the plastic encapsulation for cosmetic reasons. However, the tin/lead coat cannot reside too far inside the encapsulation, specifically within the areas where wirebonding will occur. Otherwise, the tin/lead may contaminate and disturb the delicate metallurgy of the wirebond to lead frame contact welds, rendering them unreliable. Therefore, a light registration requirement is created for the tin/lead boundary to lie inside of the plastic encapsulation, primarily a cosmetic concern, yet far enough away from the wirebonding areas so as not to contaminate them. Many perfectly functioning packaged integrated circuits are discarded for merely failing the cosmetic requirement by having a visibly exposed tin/lead boundary.
A new method for preparing solderable lead improving on the manufacturability of PPF-3000, while simultaneously overcoming the inherent solderability problems of the TI manufacturing scheme would be a great advance. Namely, the new method should be a continuous pre-plating process having good manufacturability. It should create lead frames without the cosmetic requirements of PPF-3000, provide good solderability of the external leads without the use of highly corrosive fluxes, and permit easy ultrasonic welding of bonding wires to the internal leads. This method should also be flexible enough to incorporate the use of Lead-free eletroplateable solders to permit removal of lead from the manufacturing process and the finished lead frames.